Public key cryptosystems are globally deployed on the World Wide Web, as well as on a growing number of enterprise networks, for establishment of secure communication channels. Every user in a public key cryptosystem has a pair of keys including a public key and a private key. The public key is disclosed to other users while the private key is kept secret. A public key cryptosystem typically has a primary designed use, such as for encryption, digital signature, or key agreement. Public key cryptosystems are also used for user authentication. For example, a user can authenticate itself to other users by demonstrating knowledge of its private key, which other users can verify using the corresponding public key.
In an application of a public key cryptosystem for authenticating a user, the public key must be securely associated with the identity of the user that owns the public key by authenticating the public key itself. Public key certificates are typically employed to authenticate the public key. A traditional public key certificate is a digital document, signed by a certificate authority, that binds a public key with one or more attributes that uniquely identify the owner of the public key. The public key certificate can be verified using the certificate authority's public key, which is assumed to be well known or is recursively certified by a higher authority. For example, in a corporation, a public key certificate can bind a public key to an employee number.
A public key infrastructure (PKI) refers to the collection of entities, data structures, and procedures used to authenticate public keys. A traditional PKI comprises a certificate authority, public key certificates, and procedures for managing and using the public key certificates.
One type of a user of a PKI owns the public key contained in a public key certificate and uses the certificate to demonstrate the user's identity. This type of user is referred to as the subject of the certificate or more generally as the subject. Another type of user relies on a public key certificate presented by another user to verify that the other user is the subject of the certificate and that the attributes contained in the certificate apply to the other user. This type of user that relies on the certificate is referred to as a verifier or relying party.
The association between a public key and an identity can become invalid because the attributes that define the identity no longer apply to the owner of the public key, or because the private key that corresponds to the public key has been compromised. A PKI typically employs two complementary techniques for dissociating a public key from an identity. In the first technique, each public key certificate has a validity period defined by an expiration date, which is a substantial period from the issue date, such as one year from the issue date. In the second technique, the certificate authority revokes a public key certificate if the public key certificate's binding becomes invalid before the expiration date. One way of revoking a public key certificate is by including a serial number of the public key certificate in a certificate revocation list (CRL), which is signed and issued by the certificate authority at known periodic intervals, such as every few hours or once a day. An entity that relies on a certificate is responsible for obtaining the latest version of the CRL and verifying that the serial number of the public key certificate is not on the list.
CRLs typically become quite long very quickly. When the CRLs become long, performance is severely impacted. First, CRL retrieval consumes large amounts of network bandwidth. Second, each application has a retrieve the CRL periodically, parse the CRL, and allocate storage for the CRL. Then, the application needs to carry out a linear search of the CRL for the public key certificate serial number when the application verifies each public key certificate. As a result, conventional PKIs do not scale beyond a few thousand users.
One solution proposed to alleviate the linear search problem is to partition CRLs. The serial number of the public key certificate determines where the CRL partition is located when the public key certificate is revoked. With partitioned CRLs, the application still has to retrieve and store the entire CRL or else the application needs to fetch a CRL partition in order to verify a certificate. Since certificate verification is a likely critical path, fetching a CRL partition impacts the time it takes to run the application.
An on-line certificate status protocol (OCSP) operates by permitting the verifier of the public key certificate to ask the certificate authority if the certificate is currently valid. The certificate authority responds with a signed statement. The OCSP allows CRLs to be avoided, but requires the verifier to query the certificate authority as part of the transaction that employs the public key certificates. The verifier querying the certificate authority increases the time it takes to perform the transaction. The OCSP scheme is highly vulnerable to a denial-of-service attack, where the attacker floods the certificate authority with queries. Responding to each query is computationally expensive, because each response requires a digital signature.
In a certificate status proofs scheme, the certificate authority maintains a data structure describing the set of valid and invalid certificates in the directory. For every public key certificate that has not yet expired, a short cryptographic proof can be extracted from the data structure of the certificate's current status (i.e., valid or invalid). A CRL can essentially be viewed as a cryptographic proof of invalidity for the public key certificates in the CRL, and proof of validity for those not in the CRL. The CRL, however, is not a short proof. The short cryptographic proof can be obtained by the verifier from the directory, or it can be obtained by the subject and presented to the verifier together with the public key certificate.
The Simple Public Key Infrastructure (SPKI) working group of the Internet Society and the Internet Engineering Task Force has proposed the possibility of employing short-lived certificates as a method of achieving fine-grain control over the validity interval of a certificate. See C. M. Ellison, B. Frantz, B. Lampson, R. Rivest, B. M. Thomas and T. Ylonen, SPKI Certificate Theory, Request for Comments 2560 of the Internet Engineering Task Force, September 1999. The SPKI certificate theory reference states that there are cases in which a short-lived certificate requires fewer signatures and less network traffic than various on-line test options. The user of a short-lived certificate always requires fewer signature verifications than the use of a certificate plus on-line test result.
Nevertheless, no practical method of issuing short-lived certificates has been proposed. Traditional certificates are issued off-line, as part of a process that includes subject registration, at the rate of one per year per user. By contrast, short-lived certificates would have to be issued on-line at the rate of at least one per day per user, and perhaps as often as one every few minutes for every user.
The term on-line and the term off-line have particular definitions in the context of a PKI. The term on-line herein refers to the day-to-day usage of public key certificates and key pairs for authentication. The term off-line herein refers to the more infrequent establishment or dissolution of public key bindings, which may result in the issuing or revocation of public key certificates. For example, the traditional certificate authority is off-line, issues CRLs off-line, and places the CRLs in a directory for on-line retrieval. The scheme involving certificate status proofs also makes use of off-line certificate authorities. The OCSP is the only scheme described above that employs an on-line certificate authority.
For reasons stated above and for other reasons presented in greater detail in the Description of the Preferred Embodiment section of the present specification, there is a need for an improved lightweight PKI that overcomes the above-described revocation problems and is more efficient and more scalable (in terms of numbers of certificates) than conventional PKIs.